Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
OK, this looks easy, I already put a 75 ohm termination on the coax, but I''l go back and calibrate with the 75ohm at the NanoVna output. ????????????????????????????????????? Thanks, Mikek On 7/1/2021 9:17 AM, Jim Lux wrote: On 7/1/21 6:41 AM, Zack Widup wrote:
I wouldn't sweat all these matching transformers, etc.? 75 isn't far
from 50, so you're not going to have a "math and measurement"
precision issue.
What I would do is get your 75 ohm cable, Calibrate using 50 ohm load,
short, and open at the *end* of the cable and the fact that it's a 75
ohm cable is absorbed into the calibration.
Then, if you want to know VSWR or S11 for a 75 ohm system, convert
your 50 ohm data to a 75 ohm reference impedance using math.
OR - get a 75 ohm termination, do the cal using 75, short and open,
and your data will all be with reference to a 75 ohm system.? S11 and
VSWR will be correct. What might not be correct is the resistance in
the .s1p file and the reference impedance at the center of the Smith
chart.
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Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Precision not necessary, just trying to get my feet wet again. I'm comparing a couple of DIY TV antennas and thought while I was comparing signal strength, I'd look at impedance. On 7/1/2021 9:14 AM, Larry Dodd via groups.io wrote: Yes building or buying a 75 to 50 ohm matching transformer if that¡¯s what you need (MiniCircuts sells them) is possible but it will cause a small insertion loss that needs to be accounted for if a precision measurement is needed. You didn¡¯t say if precision is necessary or just a comparison is the goal.
Larry
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Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
On Thu, Jul 1, 2021 at 07:54 AM, Donald S Brant Jr wrote:
Mikek: Unless I am misunderstanding your test intentions, a network analyzer
will not give you any signal strength indication. You would need a measuring
receiver or a spectrum analyzer to check out signal levels.
The antenna impedance and its gain/pattern/signal strength are not really
related.
73, Don N2VGU.
Best answer!! Roger
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Most TV's this "SIGNAL STRENGTH" is not a true signal strength but a signal to noise measurement.
You could have a very weak signal but if it's channel is free and clear of any noise, co-channel stations, cell phone, etc. interference, it could show a better signal strength than a station that is ten times stronger, but is plagued with all those interference issues.
Joe WB9SBD
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On 7/1/2021 11:41 AM, Fred, W9KEY wrote: Well I suppose you are looking to have fun with your NanoVNA --- but there is likely a much easier way to directly accomplish your goal.
Many modern TV's (with digital tuners) have a diagnostic mode that will display received signal strength (along with other parameters).
So look up your model on the internet, or interrogate the internal Help File, or even check the User Manual.
Using that mode will allow direct comparison of your various antennas.
For example, many Sony TV's use something like this:
And I recall the (now decades old) Digital TV Converter boxes included the same feature.
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Remember the 1/4-wavelength 'solution' is a narrowband match. Sweeping
from 180 through 610 MHz will require a number of 1/4-wavelength lines.
Not a good solution for swept measurements.
The SWR of a 75-ohm source in a 50-ohm system is only 1.5:1. That
transversing your 70-foot length of RG-6 will not produce any significant
'bad' results with a w/c SWR of 2:1 at multiple positions along the length
of RG-6. Just cal and measure in a 50-ohm system realizing that the 'bulls
eye' of the Smith Chart is now at 75-ohms which is on the single central
horizontal line and half way between 50-ohms (the 'bulls eye') and the
100-ohm circle to the right where it intersects the single central
horizontal line.
Most 'distant' TV antennas are generally of a LPDA (Log Periodic Dipole
Array) design and circle the 50 or 75-ohm point with a radius of 2:1, much
like a well designed and built discone antenna.
Dave - W?LEV
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On Thu, Jul 1, 2021 at 12:39 PM Zack Widup <w9sz.zack@...> wrote: The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote:
Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
Mikek
--
*Dave - W?LEV*
*Just Let Darwin Work*
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Well I suppose you are looking to have fun with your NanoVNA --- but there is likely a much easier way to directly accomplish your goal.
Many modern TV's (with digital tuners) have a diagnostic mode that will display received signal strength (along with other parameters).
So look up your model on the internet, or interrogate the internal Help File, or even check the User Manual.
Using that mode will allow direct comparison of your various antennas.
For example, many Sony TV's use something like this:
And I recall the (now decades old) Digital TV Converter boxes included the same feature.
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
This is what I do to measure 75 ohm loads
- build or buy a 50 to 75 ohm minimum losspad. Install on S11 port. My 75 ohm end has a F connector.
- fabricate a 75 ohm O S L kit. Calibrate using this kit. Your measurement plane is at the F connector
- measure your 75 ohm load.
Mike N2MS
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On 07/01/2021 8:38 AM Zack Widup < w9sz.zack@... > wrote:
The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek < amdx@... > wrote:
Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F
adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
Mikek
|
Re: Measuring 49:1 ferrite transformers
Very nice writeup Gary!
I may have missed it, but curious about how using two FT240 cores stacked
will behave vs total turns.
I built one of these transformers with two FT240 type 43 cores stacked,
with 2:14 turns. Do you think that will behave as well in the low-mid
frequency ranges vs. a single core with 3:21 turns?
And based on the "two-edged sword" aspect of the compensation capacitor,
would you recommend not using it, if antenna won't be used above 20m band?
I'm mainly only interested in using antenna on 20m, 40m and 80m right now,
with max power of 100W, mainly for CW and SSB.
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Mikek: Unless I am misunderstanding your test intentions, a network analyzer will not give you any signal strength indication. You would need a measuring receiver or a spectrum analyzer to check out signal levels.
The antenna impedance and its gain/pattern/signal strength are not really related.
73, Don N2VGU.
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Here is a link that has all you need to know for 75 ohm NanoVna testing.
Larry, K4LED
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On Jul 1, 2021, at 10:17 AM, Larry Dodd <ledodd@...> wrote:
?Joe, yes you are correct. All he needs is a 75 ohm resistor or 75 ohm load to re-calibrate his NanoVna to 75 ohms.
Thanks Joe.
Larry, K4LED
On Jul 1, 2021, at 10:14 AM, Joe WB9SBD <nss@...> wrote:
?I do not understand why a good 75 ohm calibration load would not make the thing be a 75 ohm test.
Isn't that what the 50 ohm one is doing when you calibrate the dead short and the 50 ohms load?
Joe WB9SBD
On 7/1/2021 8:41 AM, Zack Widup wrote: I know. That's a problem in this application. Another possibility is a
broadband impedance matching transformer with the appropriate ferrite
toroid. I calculate 12 turns on the 75 ohm side to 10 turns on the 50 ohm
side as close enough.
73. Zack W9SZ
On Thu, Jul 1, 2021 at 8:15 AM KENT BRITAIN <WA5VJB@...> wrote:
Quarter wave transformer for what frequency????
54 MHz or 600 MHz. Can't have both!
73 Kent WA5VJB
Minicircuits does sell broad band 50-75 Ohm transformers.
On Thursday, July 1, 2021, 07:39:00 AM CDT, Zack Widup <
w9sz.zack@...> wrote:
The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote: Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
Mikek
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
The more windings, the more critical the parasitice become, and
the less flat the transfer. Note that you can also buy ferrites that allow
for half a winding, or even a quarter. Ferriet Varkensneus 13,8mm
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On 1-7-2021 16:21, KENT BRITAIN wrote: MiniCircuits uses one inductor.? ?Common ground on the bottom, 75 Ohms at the top, and a tap for the 50 Ohm port.
On Thursday, July 1, 2021, 09:02:36 AM CDT, Gert Gremmen <cetest@...> wrote:
Normally the difference between 50 and 70 is neglectable. If you really
know that
the source/load is exactly 50 Ohm (that means you have measured that)
and you
really know the other side is exactly 70 Ohm, only then it's worth do do
the effort.
build a transformer on ferrite material suitable for the frequency.
You have to know that? the impedance increases with the square of the
winding ratio.
A 2 winding to 4 winding transforms your 50 into 200 Ohm.
you will need a winding ratio of 1.25 for 50-70 ohm.
At low frequency you may want a lot of windings, at high frequencies you
need much less.
Using whole windings that means that the lowest number of windings is 4 :5
This seems suitable up to 250 MHz approx with the right ferrite.
But at these high frequencies parasitic? and L may jeopardize the
impedances at both side,
normally not a big problem, but you wantend to be accurate isn't it ?
If it is narrow band, within the resonance band of an LC, you may use
a capacitor step up construction?? (C1- C2 and L back to C1)
If you connect 50 ohm over C2 you will have 70 ohm over C1+C2
if C2 = 0,25 x C1.? And the loop needs to be in resonance too.
And your caps need to be 5% or better. Quite a challenge.
Other may have even better suggestions.
Gert
On 1-7-2021 15:41, Zack Widup wrote:
I know. That's a problem in this application. Another possibility is a
broadband impedance matching transformer with the appropriate ferrite
toroid. I calculate 12 turns on the 75 ohm side to 10 turns on the 50 ohm
side as close enough.
73. Zack W9SZ
On Thu, Jul 1, 2021 at 8:15 AM KENT BRITAIN <WA5VJB@...> wrote:
? Quarter wave transformer for what frequency????
54 MHz or 600 MHz.? ? Can't have both!
73? Kent WA5VJB
Minicircuits does sell broad band 50-75 Ohm transformers.
? ? ? On Thursday, July 1, 2021, 07:39:00 AM CDT, Zack Widup <
w9sz.zack@...> wrote:
? The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote:
Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
? ? ? ? ? ? ? ? ? ? Mikek
--
Independent Expert on CE marking
EMC Consultant
Electrical Safety Consultant
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
MiniCircuits uses one inductor.? ?Common ground on the bottom, 75 Ohms at the top, and a tap for the 50 Ohm port.
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On Thursday, July 1, 2021, 09:02:36 AM CDT, Gert Gremmen <cetest@...> wrote: Normally the difference between 50 and 70 is neglectable. If you really know that the source/load is exactly 50 Ohm (that means you have measured that) and you really know the other side is exactly 70 Ohm, only then it's worth do do the effort. build a transformer on ferrite material suitable for the frequency. You have to know that? the impedance increases with the square of the winding ratio. A 2 winding to 4 winding transforms your 50 into 200 Ohm. you will need a winding ratio of 1.25 for 50-70 ohm. At low frequency you may want a lot of windings, at high frequencies you need much less. Using whole windings that means that the lowest number of windings is 4 :5 This seems suitable up to 250 MHz approx with the right ferrite. But at these high frequencies parasitic? and L may jeopardize the impedances at both side, normally not a big problem, but you wantend to be accurate isn't it ? If it is narrow band, within the resonance band of an LC, you may use a capacitor step up construction?? (C1- C2 and L back to C1) If you connect 50 ohm over C2 you will have 70 ohm over C1+C2 if C2 = 0,25 x C1.? And the loop needs to be in resonance too. And your caps need to be 5% or better. Quite a challenge. Other may have even better suggestions. Gert On 1-7-2021 15:41, Zack Widup wrote: I know. That's a problem in this application. Another possibility is a
broadband impedance matching transformer with the appropriate ferrite
toroid. I calculate 12 turns on the 75 ohm side to 10 turns on the 50 ohm
side as close enough.
73. Zack W9SZ
On Thu, Jul 1, 2021 at 8:15 AM KENT BRITAIN <WA5VJB@...> wrote:
? Quarter wave transformer for what frequency????
54 MHz or 600 MHz.? ? Can't have both!
73? Kent WA5VJB
Minicircuits does sell broad band 50-75 Ohm transformers.
? ? ? On Thursday, July 1, 2021, 07:39:00 AM CDT, Zack Widup <
w9sz.zack@...> wrote:
? The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote:
Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
? ? ? ? ? ? ? ? ? ? Mikek
-- Independent Expert on CE marking EMC Consultant Electrical Safety Consultant
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Joe, yes you are correct. All he needs is a 75 ohm resistor or 75 ohm load to re-calibrate his NanoVna to 75 ohms.
Thanks Joe.
Larry, K4LED
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On Jul 1, 2021, at 10:14 AM, Joe WB9SBD <nss@...> wrote:
?I do not understand why a good 75 ohm calibration load would not make the thing be a 75 ohm test.
Isn't that what the 50 ohm one is doing when you calibrate the dead short and the 50 ohms load?
Joe WB9SBD
On 7/1/2021 8:41 AM, Zack Widup wrote:
I know. That's a problem in this application. Another possibility is a
broadband impedance matching transformer with the appropriate ferrite
toroid. I calculate 12 turns on the 75 ohm side to 10 turns on the 50 ohm
side as close enough.
73. Zack W9SZ
On Thu, Jul 1, 2021 at 8:15 AM KENT BRITAIN <WA5VJB@...> wrote:
Quarter wave transformer for what frequency????
54 MHz or 600 MHz. Can't have both!
73 Kent WA5VJB
Minicircuits does sell broad band 50-75 Ohm transformers.
On Thursday, July 1, 2021, 07:39:00 AM CDT, Zack Widup <
w9sz.zack@...> wrote:
The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote: Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
Mikek
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Yes building or buying a 75 to 50 ohm matching transformer if that¡¯s what you need (MiniCircuts sells them) is possible but it will cause a small insertion loss that needs to be accounted for if a precision measurement is needed. You didn¡¯t say if precision is necessary or just a comparison is the goal.
Larry
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On Jul 1, 2021, at 10:03 AM, Gert Gremmen <cetest@...> wrote:
?Normally the difference between 50 and 70 is neglectable. If you really know that
the source/load is exactly 50 Ohm (that means you have measured that) and you
really know the other side is exactly 70 Ohm, only then it's worth do do the effort.
build a transformer on ferrite material suitable for the frequency.
You have to know that the impedance increases with the square of the winding ratio.
A 2 winding to 4 winding transforms your 50 into 200 Ohm.
you will need a winding ratio of 1.25 for 50-70 ohm.
At low frequency you may want a lot of windings, at high frequencies you need much less.
Using whole windings that means that the lowest number of windings is 4 :5
This seems suitable up to 250 MHz approx with the right ferrite.
But at these high frequencies parasitic and L may jeopardize the impedances at both side,
normally not a big problem, but you wantend to be accurate isn't it ?
If it is narrow band, within the resonance band of an LC, you may use
a capacitor step up construction (C1- C2 and L back to C1)
If you connect 50 ohm over C2 you will have 70 ohm over C1+C2
if C2 = 0,25 x C1. And the loop needs to be in resonance too.
And your caps need to be 5% or better. Quite a challenge.
Other may have even better suggestions.
Gert
On 1-7-2021 15:41, Zack Widup wrote:
I know. That's a problem in this application. Another possibility is a
broadband impedance matching transformer with the appropriate ferrite
toroid. I calculate 12 turns on the 75 ohm side to 10 turns on the 50 ohm
side as close enough.
73. Zack W9SZ
On Thu, Jul 1, 2021 at 8:15 AM KENT BRITAIN <WA5VJB@...> wrote:
Quarter wave transformer for what frequency????
54 MHz or 600 MHz. Can't have both!
73 Kent WA5VJB
Minicircuits does sell broad band 50-75 Ohm transformers.
On Thursday, July 1, 2021, 07:39:00 AM CDT, Zack Widup <
w9sz.zack@...> wrote:
The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote: Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
Mikek
--
Independent Expert on CE marking
EMC Consultant
Electrical Safety Consultant
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Or $2, plus shipping, from Minicircuits.
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On Thursday, July 1, 2021, 08:42:02 AM CDT, Zack Widup <w9sz.zack@...> wrote: I know. That's a problem in this application. Another possibility is a broadband impedance matching transformer with the appropriate ferrite toroid. I calculate 12 turns on the 75 ohm side to 10 turns on the 50 ohm side as close enough. 73. Zack W9SZ On Thu, Jul 1, 2021 at 8:15 AM KENT BRITAIN <WA5VJB@...> wrote: ? Quarter wave transformer for what frequency????
54 MHz or 600 MHz.? ? Can't have both!
73? Kent WA5VJB
Minicircuits does sell broad band 50-75 Ohm transformers.
? ? On Thursday, July 1, 2021, 07:39:00 AM CDT, Zack Widup <
w9sz.zack@...> wrote:
? The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote:
Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
? ? ? ? ? ? ? ? ? ? Mikek
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
On 7/1/21 7:13 AM, Joe WB9SBD wrote: I do not understand why a good 75 ohm calibration load would not make the thing be a 75 ohm test.
Isn't that what the 50 ohm one is doing when you calibrate the dead short and the 50 ohms load? exactly. I'd do the cal at the end of the coax, too.
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
On 7/1/21 6:41 AM, Zack Widup wrote:
I wouldn't sweat all these matching transformers, etc.? 75 isn't far from 50, so you're not going to have a "math and measurement" precision issue.
What I would do is get your 75 ohm cable, Calibrate using 50 ohm load, short, and open at the *end* of the cable and the fact that it's a 75 ohm cable is absorbed into the calibration.
Then, if you want to know VSWR or S11 for a 75 ohm system, convert your 50 ohm data to a 75 ohm reference impedance using math.
OR - get a 75 ohm termination, do the cal using 75, short and open, and your data will all be with reference to a 75 ohm system.? S11 and VSWR will be correct. What might not be correct is the resistance in the .s1p file and the reference impedance at the center of the Smith chart.
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
I do not understand why a good 75 ohm calibration load would not make the thing be a 75 ohm test.
Isn't that what the 50 ohm one is doing when you calibrate the dead short and the 50 ohms load?
Joe WB9SBD
toggle quoted message
Show quoted text
On 7/1/2021 8:41 AM, Zack Widup wrote: I know. That's a problem in this application. Another possibility is a
broadband impedance matching transformer with the appropriate ferrite
toroid. I calculate 12 turns on the 75 ohm side to 10 turns on the 50 ohm
side as close enough.
73. Zack W9SZ
On Thu, Jul 1, 2021 at 8:15 AM KENT BRITAIN <WA5VJB@...> wrote:
Quarter wave transformer for what frequency????
54 MHz or 600 MHz. Can't have both!
73 Kent WA5VJB
Minicircuits does sell broad band 50-75 Ohm transformers.
On Thursday, July 1, 2021, 07:39:00 AM CDT, Zack Widup <
w9sz.zack@...> wrote:
The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote:
Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
Mikek
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Zack,
12/10 turns is 2% off - certainly close enough as you say, but 11/9 turns is even better at 0.2% off.
73/72
John
M0JBA
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On 1 Jul 2021, at 14:41, Zack Widup <w9sz.zack@...> wrote:
I know. That's a problem in this application. Another possibility is a
broadband impedance matching transformer with the appropriate ferrite
toroid. I calculate 12 turns on the 75 ohm side to 10 turns on the 50 ohm
side as close enough.
73. Zack W9SZ
On Thu, Jul 1, 2021 at 8:15 AM KENT BRITAIN <WA5VJB@...> wrote:
Quarter wave transformer for what frequency????
54 MHz or 600 MHz. Can't have both!
73 Kent WA5VJB
Minicircuits does sell broad band 50-75 Ohm transformers.
On Thursday, July 1, 2021, 07:39:00 AM CDT, Zack Widup <
w9sz.zack@...> wrote:
The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote:
Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
Mikek
|
Re: impedance test of a TV antenna, How do I adjust 50 ohm to 75 ohm
Normally the difference between 50 and 70 is neglectable. If you really know that
the source/load is exactly 50 Ohm (that means you have measured that) and you
really know the other side is exactly 70 Ohm, only then it's worth do do the effort.
build a transformer on ferrite material suitable for the frequency.
You have to know that? the impedance increases with the square of the winding ratio.
A 2 winding to 4 winding transforms your 50 into 200 Ohm.
you will need a winding ratio of 1.25 for 50-70 ohm.
At low frequency you may want a lot of windings, at high frequencies you need much less.
Using whole windings that means that the lowest number of windings is 4 :5
This seems suitable up to 250 MHz approx with the right ferrite.
But at these high frequencies parasitic? and L may jeopardize the impedances at both side,
normally not a big problem, but you wantend to be accurate isn't it ?
If it is narrow band, within the resonance band of an LC, you may use
a capacitor step up construction?? (C1- C2 and L back to C1)
If you connect 50 ohm over C2 you will have 70 ohm over C1+C2
if C2 = 0,25 x C1.? And the loop needs to be in resonance too.
And your caps need to be 5% or better. Quite a challenge.
Other may have even better suggestions.
Gert
toggle quoted message
Show quoted text
On 1-7-2021 15:41, Zack Widup wrote: I know. That's a problem in this application. Another possibility is a
broadband impedance matching transformer with the appropriate ferrite
toroid. I calculate 12 turns on the 75 ohm side to 10 turns on the 50 ohm
side as close enough.
73. Zack W9SZ
On Thu, Jul 1, 2021 at 8:15 AM KENT BRITAIN <WA5VJB@...> wrote:
Quarter wave transformer for what frequency????
54 MHz or 600 MHz. Can't have both!
73 Kent WA5VJB
Minicircuits does sell broad band 50-75 Ohm transformers.
On Thursday, July 1, 2021, 07:39:00 AM CDT, Zack Widup <
w9sz.zack@...> wrote:
The best way would be to use a quarter-wave section of 62 ohm cable.
Lacking that, you may have to build some sort of antenna tuner.
73, Zack W9SZ
On Thu, Jul 1, 2021 at 7:13 AM Mikek <amdx@...> wrote:
Testing a couple of TV antennas for Signal strength and just curious about
what they might look at on the NanoVna.
I'll be looking at them through about 70 ft of RG-6 and a SMA to F adapter.
Measuring from 180MHz to 610MHz.
Pretty much a novice here.
Mikek
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